The invention relates to an optoelectronic sensor having at least two pixels wherein each pixel has a diode-based photodetector for generating an electric photocharge as a function of incident electromagnetic radiation, and means for electrically connecting at least two pixels, wherein the connecting means includes means (6) for switching the connection on or off. The invention further relates to a method for detecting electromagnetic radiation using such an optoelectronic sensor, including the steps of generating an electric photocharge as a function of incoming electromagnetic radiation, and electrically connecting at least two pixels.
To date, two principles for solid-state image sensors prevail: charge-coupled-device (CCD) sensors and active-pixel sensors (APS). CCD sensors need a special manufacturing technology and have a relatively high power consumption. On the other hand, with CCD image sensors it is possible to increase the light sensitivity by concentrating in one single pixel the electric charge generated in several pixels. By doing this, the spatial resolution is reduced, but also the readout time and the power consumption are reduced. Thus CCD sensors can exploit the photocharge detection sensitivity of one pixel and the photocharge collection capability of many adjacent pixels. This operation mode, which is called xe2x80x9ccharge binningxe2x80x9d, is described, e.g., in P. Seitz et al., xe2x80x9cSmart optical and image sensors fabricated with industrial CMOS/CCD semiconductor processesxe2x80x9d, SPIE Vol. 1900, pp. 21-30, 2-3 February 1993. However, the readout time, or the power consumption (for a constant frame frequency), cannot be reduced by the same factor as the spatial resolution, because for a readout it is necessary to drive the whole sensor.
APS sensors have the advantage of being realizable with standard complementary metal oxide semiconductor (CMOS) processes and of showing a relatively low power consumption. In an APS sensor, in each pixel the charge or current generated by the photodetector is actively converted into a voltage or current signal. Each pixel can be addressed and read out individually. It is possible with APS sensors to increase the readout velocity and/or the power consumption by reducing the spatial resolution. To date, it was however not possible to increase the light sensitivity of a pixel by interconnecting several pixels of an APS sensor. Of course, it is possible to connect CMOS pixels with suitably placed MOS-FET switches to form one big xe2x80x9csuper-pixerxe2x80x9d. Although the effective photocharge collection area is enlarged in this way, the CMOS pixels find themselves all connected in parallel, enlarging the total capacitance of the xe2x80x9csuper-pixelxe2x80x9d. No mechanism has been known to force the distributed photocharge from the xe2x80x9csuper-pixelxe2x80x9d into one CMOS pixel for sensitive charge detection in this pixel.
It is therefore an aim of the present invention to provide an optoelectronic sensor that comprises diode-based photodetectors and is suited for photocharge binning. It is a further aim of the invention to provide a method for detecting light with diode-based photodetectors that make photocharge binning possible.
The invention encompasses:
a suitable topology of, preferably, CMOS pixels (with photodiode, reset gate and charge detector circuit) which are interconnected with MOS-FET switches controlled from the outside;
special charge detection circuits which have an additional MOS-FET (xe2x80x9cskimmerxe2x80x9d) at their inputs, offering a floating source and floating drain; and
an operation mode in which the MOS-FET switches can be controlled to interconnect the pixels, and the skimming MOS-FET""s gate is set to a DC potential which lies below the charge detection circuit""s reset voltage.
The optoelectronic sensor according to the invention comprises at least two pixels, each pixel having a diode-based photodetector for generating an electric photocharge as a function of incident electromagnetic radiation, and means for electrically connecting at least two pixels. The connecting means include means for switching the connection on or off. The pixels are designed such that, if at least two pixels are connected, the photocharges generated in said connected pixels are combined in one of the connected pixels, whereby the spatial resolution of the sensor is reduced.
The method according to the invention for detecting electromagnetic radiation using the optoelectronic sensor according to the invention comprises the steps of generating an electric photocharge as a function of incoming electromagnetic radiation, and electrically connecting at least two pixels. The photocharges generated in the connected pixels are combined in one of the connected pixels, whereby the spatial resolution of the sensor is reduced.
In this document, the term xe2x80x9clightxe2x80x9d is used for any kind of electromagnetic radiation, such as visible light, infrared (IR) or ultraviolet (UV) radiation.
The invention makes it possible to vary the spatial resolution, the light sensitivity and/or the readout velocity by purely electronic means. The sensor can be manufactured in any CMOS technology without adding modified or additional steps. The sensors total readout time and/or power consumption can be reduced by reducing the spatial resolution. In the sensor, groups of pixels are preferably isolated from each other by a depleted transistor channel, which may also be switched into a conductive state, thus increasing the light sensitivity of a xe2x80x9csuper-pixelxe2x80x9d composed of several pixels. Thus, in the image sensor according to the invention the light sensitivity or the spatial resolution can be varied purely electronically.
Reducing the spatial resolution by connecting pixels according to the invention has the advantage of an increased light sensitivity, and, moreover, of being offset free because the signal of each pixel is read out via the same sensing node. This does not hold if the reduction of the resolution is only carried out by averaging after reading out
The power consumption of the sensor is reduced at a constant frame frequency if the spatial resolution is reduced, because only those pixels have to be supplied with voltage which effectively have to be read out.
The invention also encompasses an architecture of control lines for controlling connecting switches between pixels in a two-dimensional sensor. With this architecture, square fields with 22n(=4, 16, 64, . . . pixels, where n=1, 2, 3, . . . is an integer, can be formed by control lines oriented in one single direction. The invention further encompasses an architecture of the above-mentioned control lines which makes it possible to form rectangular fields with pxq pixels, where p=1, 2, . . . and q=1, 2, . . . are integers.